Petrol and diesel pump prices dropping as low as 90p per litre is 'not that far-fetched', according to fuel-price experts after the four main supermarkets cut the price of diesel below £1 this week.

Asda, Morrisons, Sainsbury's and Tesco all slashed the price of diesel on Monday, a month after the four retailers competed for motorists' custom by offering petrol for under £1 a litre.

But the savings for car drivers might not end there. The RAC said a recent announcement from Saudi Arabia confirming its commitment to oil production could see a continuation of oversupply and keep fuel below £1 for the early part of the year - predicting prices could plummet another 10p.

A sign of things to come: The RAC says fuel prices should remain below £1 a litre for the early part of 2016

Morrisons was the first retailer to offer diesel for less than £1, reducing the cost to 99.7p per litre nationwide on Sunday.

Asda followed suit by hacking 3p off the price of the fuel on at its 279 filling stations on Monday — 24 days after it did the same to petrol prices — with Tesco matching the price at its 500 fuel locations.

Sainsbury's said it would also be offering drivers some New Year's savings by also cutting diesel to less than £1 a litre at its 301 forecourts on Monday, but is charging motorists 99.9p.

The average price of petrol fell by more than 4p a litre in December, according to RAC Fuel Watch — from 107.55p at the start to 102.89p by the close, reducing the cost of filling up a family-sized 55-litre car for the sixth consecutive month and resulting in a saving of £2.02 on every tank.

This was the lowest average petrol price since December 2009 and led to a tank of petrol costing £5.35 less than it did a year ago.

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It's the first time in more than six years that diesel has been dropped below the £1 marker, and experts have said the lower cost of filling up won't be short-lived with crude oil prices expected to remain low.

The RAC's Fuel Watch, which monitors the price UK drivers pay at filling stations, predicted that petrol and diesel would remain below £1 for the opening period of 2016, and overproduction of oil could see prices fall another 10p per litre despite fuel duty making up the majority of the pump costs.

The lion's share of the price we pay at the pump, around 75%, goes on duty and VAT

The average price of diesel dropped 3p a litre during December — from 109.86p to 106.18p — cutting the cost of filling a typical tank by £2.56 and making it £6.91 cheaper than at the end of December 2014 and is expected to fall further now retailers have cut prices below £1.

RAC spokesman Pete Williams said: 'The lion's share of the price we pay at the pump, around 75 per cent, goes to the Treasury in duty and VAT. The rest of the price is production, refinery costs, distribution and the fuel retailers' margins.

'This means that there is a limit to how low we will see the price of diesel and petrol go, but prices could well continue to fall and a pump price of 90p a litre is not that far-fetched.'

Williams added that the drop below £1 for diesel pump prices was long overdue considering wholesale diesel costs have been lower than petrol and crude oil prices have been below $40 a barrel for a prolonged period, falling as low as $36 a barrel on December 21 — the cheapest it has been for 11 years.

'Diesel drivers will clearly welcome this move by the big supermarkets, although it would be fair to say it has been slow in coming,' Williams said.

Average fuel prices for 2015 shows the gap between the wholesale price of petrol and diesel against the amount motorists are charged at the pumps

The RAC's two-week forecast says diesel is likely to come down by more than 4p per litre in the next fortnight, though petrol prices are expected to remain consistent due to the higher wholesale price of the fuel

'The wholesale price of diesel has been around two pence cheaper than the wholesale price of petrol since a couple of weeks before Christmas so we should have seen these cuts earlier and that is what the RAC has been calling for.

'We hope that other supermarkets and the cheaper fuel retailers will follow suit swiftly and do the right thing for motorists.

'This should reduce the average price of diesel across the UK for motorists everywhere who can then benefit from the low price of crude oil on the world commodity market.'

The RAC Foundation also questioned why there had been such a delay for diesel to drop below £1 at the pump.

Steve Gooding, director for the transport charity, said: 'The question is, why haven't we seen these bargains sooner and more widely?

'After all, the wholesale price of diesel has been below that of petrol since early December, yet the story at the pumps has been the opposite with diesel 3 or 4p a litre higher throughout that period.'

'More generally, low pump prices are a product of a barrel of oil changing hands for less than a third of the price it did 18 months ago.'

Petrol dropped below £1 a litre before Christmas and many motoring groups have been calling for retailers to do the same with diesel prices

Both diesel and petrol prices are now at the lowest they've been since 2009 and, with oil prices likely to remain low thanks to Saudi Arabia's supply, the RAC says lower costs at UK forecourts should continue based on oil prices.

'The world is awash with oil at present and the decision by the Opec oil producing countries and particularly their biggest producer, Saudi Arabia, to continue pumping oil will ensure that this oversupply continues,' Williams added.

'Combined with significantly lower demand from China and the developing nations together with the prospect of additional supplies from Iran means that there will be no let up in cheap oil.'

However, despite no mention in George Osborne's Autumn statement in November last year, fuel duty is expected to increase in the second quarter of 2016.

The small print of the Chancellor's most recent Spending Review documents shows the Treasury’s takings from fuel duty are expected to rise at the rate of inflation every year until 2020.

This could mean a rise of more than 2p a litre in April 2016 – and steeper rises every year after that.